Single crystals are used in a variety of high performance industries. For example, single crystal silicon wafers are used in the semiconductor industry and single crystal sapphire crystals are used in the defense (antenna windows) electronic (light emitting diodes) and general industries (laser scanner windows). Such single crystals are usually made in a high temperature operation.
An example of this is the production of silicon wafers for use in the semiconductor industry by the Czochralski or "CZ" process. In the CZ process, a seed crystal of known orientation is immersed in a molten pool of silicon. This triggers solidification and precipitation of the silicon. As the crystal is mechanically pulled upwardly from the pool, the orientation of the solidifying front mimics that of the seed crystal. Silicon wafers can be manufactured from the solid ingot by machining and polishing.
Specifically constructed furnaces are used to accurately control the various parameters needed to ensure that high quality crystals are produced. Several of the key components in these crystal growing furnaces are made from graphite. These include various liners, shields, tubes, crucible susceptors and the like. Graphite has been the material conventionally utilized in such processes due to its high temperature capability and relative chemical inertness.
Disadvantages of graphite include its poor durability brought about by its highly brittle nature and its tendency to microcrack when exposed to repeated temperature cycles. Such microcracking alters the thermal conductivity of the component which in turn makes accurate temperature control of the crystal melt difficult. In addition, contamination of the melt may occur by the leaching of impurities from the graphite components or from particulates generated by the degradation of the graphite itself. Semiconductor standards require extremely low levels of impurities in the semiconductor processing system, to allow substantially no impurities to be incorporated into the semiconductor material, as even trace amounts can alter the electronic properties of the semiconductor material.
Further, the deposition of oxides of silicon on graphite parts during the production of the silicon crystal occurs to such an extent that parts must be cleaned on a regular basis and replaced periodically. Replacing worn graphite parts is a time consuming and costly process.
Therefore, there has been a need for the manufacture of components for single crystal growing reactors that have the advantages of graphite without the disadvantages. Such components would enable the more cost effective production of high quality single crystals, including silicon semiconductor wafers.
There have been attempts made to utilize carbon/carbon composites in similar electronic material production processes, in place of graphite furnace components and furniture. U.S. Pat. No. 5,132,145 and corresponding European Patent application 88401031.5 to Valentian disclose a method of making a composite material crucible for use in the Bridgman method for producing single crystals of metallic material semiconductors.
Valentian proposed making a cylindrical crucible for holding a molten sample, from a single wall of carbon fibers or silicon carbide fibers impregnated with carbon or silicon carbide, and depositing on the inner wall of the crucible, a thin inner lining of silicon carbide in combination with silica, silicon nitride, and silicon nitride/alumina, or in other embodiments, amorphous carbon, boron nitride, titanium nitride or diboride, and zirconium nitride or diboride. The thin inner lining is required to avoid contamination of the molten sample, to provide a matched thermal conductivity with the molten sample, and to avoid crack propagation which is a drawback of the bulk material.
One of the most critical components in the high temperature single crystal growing furnaces is the susceptor. The function of the susceptor is to hold a crucible (usually quartz in the silicon crystal growing process) which is in intimate contact with the crystal melt. The susceptor must also allow for the transfer of heat from the heater to the crystal mass. This must be accomplished as uniformly as possible. Accurate control of the thermal environment is critical to the success in fabricating high quality single crystals.
Fabrication of graphite susceptors is not trivial. The low strength characteristics of graphite and the need to support the crystal mass, means that thick sections have to be used, particularly in the base of the crucible. These thick sections contribute to a high level of thermal mass and consequently difficulties in controlling the thermal environment accurately.
In the Czochralski (CZ) process, the current, conventional CZ crystal pulling susceptor is designed to hold the quartz crucible in place during the CZ crystal pulling operation, which in turn holds the polysilicon used to make the silicon crystal. Quartz softens at approximately 1150.degree. C. The CZ process runs at approximately 1450.degree. C. The quartz crucible softens during the CZ operation and conforms to the susceptor.
The susceptor must be able to retain its shape in an argon atmosphere at reduced pressure. It must not outgas and it must be of sufficient purity not to effect the material properties of the polysilicon that is being contained by the quartz crucible. Finally, it must have the proper thermal characteristics to allow for the correct thermodynamic conditions needed to grow a silicon crystal with minimal or zero defect dislocations. The dislocations can occur from both contamination and variation in the thermodynamic conditions within the furnace.
The current conventional CZ susceptor material is graphite. While graphite does an adequate job for the current size CZ crystal pullers and can retain its shape during the crystal pulling operation, it can however, catastrophically fail during operation. If the graphite susceptor catastrophically fails, it may lose its containment of the quartz crucible during the CZ operation. The loss of containment of the quartz crucible can have serious consequences. If the molten polysilicon comes into contact with the water cooled steel furnace vessel, catastrophic furnace failure will occur. This would include destroying the furnace and more importantly, it could cause serious injury to personnel. Some of the current furnace graphite configurations are designed specifically for the containment of the polysilicon during the crystal growing phase of the operation.
A further disadvantage of graphite susceptors stems from the need to produce the graphite component in four parts. Three pieces constitute the susceptor and the fourth piece is required for the susceptor base, which holds the susceptor together and interfaces the susceptor to the furnace pedestal. This is done because graphite does not have the toughness and impact strength required to prevent breaking during the crystal growing process. The CZ silicon crystal growing process leaves some liquid silicon metal at the conclusion of crystal growth. The remaining silicon metal expands approximately 9% upon cooling. The stress induced by the thermal expansion of the silicon metal results in the breakage of a single piece graphite susceptor. Additionally, a single piece graphite susceptor will break upon the removal of the quartz crucible.
Although machining of the graphite can be done to close tolerances, gaps still exist between the three sections of the component. By-product gases of the crystal pulling process are highly corrosive (such as silicon monoxide in the CZ process) and can attack the graphite structure through these gaps. This in turn reduces the lifetime of the components and seriously affects the crystal production rate.
Another issue involved with the graphite susceptor is thermal management. The current graphite susceptor design consists of a conical side section, generally between 0.5" to 1" in thickness. The bottom section matches the bottom contour of the quartz crucible. The bottom section matches the bottom contour of the quartz crucible. The bottom section is substantially thicker than the conical side section. Additionally, in the furnace, the susceptor sets on a graphite base. The combination of the graphite base and thick bottom section of the susceptor increases the difficulty in hot zone thermal management.
The thermal management difficulty arises from two factors. First is the non-uniformity of part thickness between the susceptor side section area and the base area. Second, the susceptor base protrudes sufficiently to shield the bottom of the susceptor from the adjacent heater. This shielding causes the heater to add more energy into the system to compensate for the additional thermal mass and thermal shielding. Therefore, there is a need to develop a one piece susceptor of low thermal mass, that exhibits durability.
It is therefore an object of the present invention to provide components for use in semiconductor processing that are superior in mechanical and thermal properties to conventional graphite components.
It is a further object of the present invention to provide components for use in semiconductor processing that are superior in purity characteristics to conventional graphite components and to conventional carbon/carbon materials.
It is a further object of the present invention to provide a carbon/carbon susceptor capable of providing complete crucible containment, thus eliminating the need for additional spill containment resources.
It is a further object of the present invention to provide a carbon/carbon susceptor capable of providing improvement in the thermal management of the CZ furnace hot zone, thus providing potential energy savings and improvement of silicon crystal quality by a reduction of crystal dislocations.
It is a further object of the present invention to provide a carbon/carbon susceptor capable of providing an increase in the hot zone for a given fixed furnace vessel or size, by a reduction in the susceptor side thickness, thus providing the CZ crystal grower with an increase in the amount of polysilicon placed in the enlarged quartz crucible.
It is a further object of the present invention to provide a carbon/carbon susceptor capable of providing the CZ crystal grower with a susceptor of greatly increased lifetime, namely an increased number of cycles prior to replacement.
It is a further object of the present invention to provide a carbon/carbon susceptor capable of production in a single piece which is tough and durable.